Miniaturization and downsizing of electronic devices continue to advance with the increasing rapid diffusion of data terminal electronic devices. Demands are concomitantly increasing for higher densities and smaller sizes for the printed circuit boards (printed wiring boards) mounted in such devices. The increasing functions of electronic devices such as cellular phones are also requiring connection between an ever greater variety of high performance modules or high-density printed circuit boards, including cameras and the like.
Such connecting materials, i.e flexible wiring board materials, must exhibit adhesion, heat resistance, flexibility, electrical insulating properties and long-term reliability. As electronic materials satisfying these demands, there have been used, specifically, resin compositions comprising curing agents added to acrylic-based resins such as acrylonitrile-butadiene-based resins or carboxy-containing acrylonitrile-butadiene resins (see Patent document 1, for example).
Acrylic-based resins have excellent features compared to other flexible resins, including (1) suitable tack strength, (2) easy introduction of functional groups and (3) transparency.
However, such resin compositions clearly have inferior ion migration resistance and insulating reliability. Ion migration is a phenomenon in which a metal composing the wiring or circuit pattern on an insulating material or inside an insulating material, or an electrode, migrates onto the insulating material or into the insulating material by differences in potential during electrification in a high humidity environment.
Ionic impurities are one cause of this phenomenon. In Patent document 2, a method of adding an inorganic ion exchanger is proposed as a countermeasure.